1. Field of the Invention
This invention relates to a press drying apparatus for removing excess water from a formed web by the simultaneous application to the web of elevated temperature and pressure for a prolonged period of time. More specifically, this invention relates to a press drying apparatus in which deflection of a heated roll is compensated for.
2. Information Disclosure Statement
In the papermaking art, excess water is removed from a formed web by passing the formed web through one or more pressing nips of a press section such that the pressed web entering the subsequent drying section requires the least amount of thermal energy in order to provide a web having the requisite moisture content. In practice, if more water can be removed during passage of the web through the press section, less heat is required in the drying section in order to obtain a resultant web having the right amount of moisture remaining. From the point of view of cost effectiveness alone, it is desirable to remove as much water as possible in the press section rather than attempting to remove it in the drying section. In view of the aforementioned objective of removing as much water as possible in the press section, proposals have been made to apply both pressure and externally-applied thermal energy to the press -section such that water within the formed web is vaporized during passage through the press drying nip. A prior art proposal of this type is disclosed in U.S. Pat. No. 4,324,613 to Wahren.
U.S. Pat. No. 4,324,613 shows a heated roll and a backing roll defining therebetween a press drying nip for passage therethrough of the formed web. The heated roll is heated by means of a burner such that the external surface of the heated roll is heated prior to rotation through the press drying nip.
Although the aforementioned proposal of U.S. Pat. No. 4,324,613 met with limited success in a laboratory application, this proposal was impractical when applied to a commercial press operation wherein web widths of up to 34 feet are commonplace. In commercial production, deflection, or bending of the press roll in a conventional or extended nip press roll, is compensated for by controlled crown roll means such that the nipping pressure applied at any point in a cross-machine direction remains substantially constant. Such crown controlled rolls typically include a central non-rotatable beam and a rotatable shell rotating around the fixed beam with a plurality of hydraulically-operated shoes disposed between the beam and the internal surface of the shell such that bowing (or deflection) of a cooperating backing roll is compensated for. Although, in conventional pressing techniques, including extended nip pressing techniques, such controlled crown rolls have met with considerable success, problems have existed when attempting to compensate for roll deflection when applying excessive heat to the heated press roll a in the press drying technique of the aforementioned U.S. Pat. No. 4,324,613.
Typically, the heated roll which may be up to 34 feet in width is heated within the range 300.degree.-700.degree. Fahrenheit. Additionally, the nip loading ranges from 300 to 6,000 pli. Such high temperatures cause serious problems when applying conventional crown control deflection techniques because the hydraulic fluid is excessively heated and oil seals tend to leak. Additionally, with the application of such excessive heat, the internal structure of such controlled crown rolls is subjected to very high thermal distortion.
The present invention overcomes the aforementioned problems by the provision of a heated roll of simple construction which is strong enough to withstand hoop stresses but which is able to bend, or deflect, under pressure. The aforementioned deflection is compensated for not by internal crown control deflection means but by the provision of a separate controlled crown roll which cooperates with the heated roll by defining a nip therebetween which is diametrically opposed to the aforementioned press drying nip.
Therefore, it is a primary objective of the present invention to provide a press drying apparatus which overcomes the aforementioned problems associated with the prior art proposals and which provides a significant contribution to the press drying art.
Another object of the present invention is the provision of a press drying apparatus which includes a heated roll and a pressing means which cooperates with the heated roll for defining therebetween a first nip for the passage therethrough of the web. A deflection controlling means cooperates with the heated roll for defining therebetween a second nip with the second nip being disposed diametrically opposite to the first nip such that when the web extends through the first nip and the heated roll is heated, deflection of the heated roll towards the second nip is compensated for by the deflection controlling means.
Another object of the present invention is the provision of a press drying apparatus in which a heated roll includes a shell having a thickness such that hoop stresses are supported while axial bending is permitted.
Another object of the present invention is the provision of a press drying apparatus in which an induction heater is disposed in close proximity to the heated roll for heating the heated roll.
Another object of the present invention is the provision of a press drying apparatus in which a pressing means includes an extended nip press shoe which defines a surface for cooperation with a heated roll, the shoe being urged towards the heated roll for defining therebetween a first nip.
Another object of the present invention is the provision of a press drying apparatus in which the deflection controlling means is a controlled crown roll.
Another object of the present invention is the provision of a press drying apparatus which includes a blow box disposed in close proximity to the controlling means such that a curtain of cooling air is blown against the controlling means to remove heat transmitted to the controlling means via the second nip.
Another object of the present invention is the provision of a press drying apparatus in which the controlling means is a second extended nip press.
Another object of the present invention is the provision of a press drying apparatus which includes a web guiding means which is disposed downstream relative to a press drying nip and upstream relative to a second calendering nip, the guiding means guiding the web from the first nip through the second nip.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings.